High-frequency mechanical wave measuring device

Abstract

High-frequency mechanical wave measurement apparatus. This apparatus comprises: - a demultiplexer (52) capable of extracting optical signals reflected by measurement Bragg gratings, this demultiplexer comprising for this purpose a series of bandpass filters (Fpi) in which each bandpass filter (Fpi) is associated with a respective measurement Bragg grating (Bmi), each of these bandpass filters having a bandwidth centered on a central wavelength and the width of this bandwidth being greater than the reflection range of the measurement Bragg grating to which it is associated, and - a servo device (130) capable of varying the central wavelength of each bandpass filter (Fpi) to cancel variations in the DC component of the optical signal reflected by the measurement Bragg grating to which it is associated. Fig. 3

Description

Title of the invention: Device for measuring high-frequency mechanical waves

[0001] The invention relates to a device for measuring high-frequency mechanical waves and to an instrumented structure comprising this measuring device.

[0002] The invention applies in particular, but not exclusively, to the field of non-destructive testing and structural health monitoring.

[0003] For example, a high-frequency acoustic wave measuring device is disclosed in the following article: ZHAO Yang et al.: “A laser-based fiber Bragg grating ultrasonic sensing system for structural health monitoring,” IEEE Photonics Technology Letters, 2016, vol. 28, no. 22, pp. 2573–2576. Hereafter, this article is referred to as “Zhao2016.” This known measuring device comprises:

[0004] - an optical fiber to be fixed to a structure to be monitored in which are carried out several Bragg gratings,

[0005] - a generator of an excitation signal formed by several laser sources which each emit an excitation beam reflected by a respective Bragg grating,

[0006] - a demultiplexer capable of extracting the optical signals reflected by each of the Bragg networks, this demultiplexer including for this purpose a series of bandpass filters, and

[0007] - a set of photodiodes capable of measuring each of the optical signals extracted by the demultiplexer.

[0008] Each of the demultiplexer's bandpass filters has a -3 dB bandwidth centered on a central wavelength. The width of this bandwidth is greater than the reflection range of a respective measurement peak of one of the Bragg gratings. The width of this bandwidth is also sufficiently narrow so as not to overlap a measurement peak of another Bragg grating.

[0009] An acoustic wave propagating through the monitored structure deforms the Bragg grating it encounters. This deformation of the Bragg grating slightly alters the fundamental wavelength XB of the Bragg grating. This change in the wavelength XB modifies the power of the optical signal reflected by the Bragg grating at a given wavelength. Thus, when environmental conditions are constant, a change in the amplitude of the reflected optical signal, measured by a photodiode, is a measure of the acoustic wave at the location of this Bragg grating.

[0010] The fundamental wavelength XB can also vary depending on the conditions in Environmental factors such as temperature or mechanical deformation of the structure can affect the measurement. Therefore, the bandwidth of each bandpass filter must be large enough to always include the XB wavelength of the corresponding Bragg grating. Thus, in each measuring device such as the one described in the Zhao2016 article, the bandwidth of each bandpass filter is sized so that, even under extreme environmental conditions, the XB wavelength always remains within that bandwidth. Consequently, the bandwidths of the demultiplexer's bandpass filters are much wider than the measurement peaks. Furthermore, since each bandwidth must contain at most one measurement peak, the wider the bandpass filter bandwidths, the greater the spacing between the XB wavelengths of the Bragg gratings.Thus, the number of Bragg gratings that can be queried by the same generator decreases as the bandwidth of the bandpass filters increases. In other words, the density of Bragg gratings decreases as the bandwidth of the bandpass filters increases.

[0011] The invention aims to provide a high-frequency acoustic wave measurement device in which, even when environmental conditions vary, a higher Bragg grating density can be achieved.

[0012] The invention therefore relates to a device for measuring high-frequency mechanical waves, that is to say, mechanical waves with frequencies greater than 1 kHz, this device comprising:

[0013] - an optical fiber in which several measurement Bragg gratings are implemented intended to be exposed to high-frequency mechanical waves, each of these measurement Bragg gratings exhibiting a reflection spectrum which includes a measurement peak centered on a measurement wavelength and whose width defines a reflection range which contains all the wavelengths reflected by this measurement peak, the measurement wavelengths of each of these Bragg gratings being different from each other and spaced from each other in such a way that the different reflection ranges of these measurement Bragg gratings are distinct and do not overlap,

[0014] - a generator capable of emitting an optical excitation signal in the optical fiber, the emission spectrum of this optical excitation signal comprising as many emission peaks as there are measurement Bragg gratings, each of these emission peaks being located, at least in part, within the reflection range of a respective measurement peak,

[0015] - a demultiplexer capable of extracting the optical signals reflected by each of the Bragg measurement networks, this demultiplexer comprising for this purpose a series of bandpass filters wherein each bandpass filter is associated with a respective measurement Bragg grating, each of these bandpass filters having a -3 dB bandwidth centered on a central wavelength and the width of this bandwidth being greater than the reflection range of the measurement peak of the measurement Bragg grating to which it is associated and sufficiently narrow so as not to encroach on a measurement peak of another measurement Bragg grating, and

[0016] - a set of photodiodes capable of measuring each of the optical signals extracted by the demultiplexer,

[0017] in which the device includes a first servo device capable of varying the central wavelength of each bandpass filter to cancel the variations of the continuous component of the optical signal reflected by the measurement Bragg grating to which it is associated.

[0018] Embodiments of this device may include one or more of the following features:

[0019] 1) Each bandpass filter comprises an optically filtering Bragg grating connected to the optical fiber to receive the optical signal to be demultiplexed and optically connected to one of the photodiodes to deliver to this photodiode the optical signal extracted by this bandpass filter, the reflection spectrum of this filtering Bragg grating having a filtering peak centered on the central wavelength of this bandpass filter and the width of this filtering peak being greater than the width of the reflection range of the measurement peak of the measurement Bragg grating associated with this bandpass filter.

[0020] 2) The first control device comprises, for each Bragg network of filtering, a transducer capable of mechanically deforming this filtering Bragg grating to modify the central wavelength of this bandpass filter.

[0021] 3) The generator comprises:

[0022] - an optical source capable of emitting incoherent light at an output, the The power spectrum includes an emission band that extends across all reflection ranges of the measurement peaks, and

[0023] - a filtering device comprising an input optically connected to the output of the optical source and an output optically connected to the optical fiber, the power spectrum of this filtering device comprising as many -3 dB bandwidths as there are measurement Bragg gratings, each of these bandwidths being centered on a respective emission peak to form this emission peak in the optical excitation signal.

[0024] 4) The filtering device comprises as many source Bragg gratings as there are measurement Bragg gratings, each source Bragg grating being associated with a respective measurement Bragg grating, the reflection spectrum of each source Bragg grating including the emission peak which is located, at least in part, within the reflection range of the measurement peak of the measurement Bragg grating associated with it.

[0025] 5) The peak width of the emission from each source Bragg grating is equal to or greater smaller than the width of the measurement peak of the measurement Bragg grating associated with it.

[0026] 6) Each emission peak is centered on a central wavelength and the device includes a second control device capable of varying the central wavelength of each source Bragg grating to cancel variations in the DC component of the optical signal reflected by the measurement Bragg grating to which it is associated.

[0027] 7) The second control device comprises, for each Bragg network source, a mechanical transducer capable of mechanically deforming this source Bragg grating to modify the central wavelength of this source Bragg grating.

[0028] 8) The first and second control devices are combined and each transducer is capable of mechanically deforming both the bandpass filter's filtering Bragg grating and the source Bragg grating that are associated with the same measurement Bragg grating.

[0029] 9) Each transducer is a piezoelectric material on which the Bragg grating The filtering and source Bragg network are fixed with no degrees of freedom.

[0030] The invention also relates to an instrumented structure comprising:

[0031] - a structure in which a defect, capable of modifying the propagation of waves High-frequency mechanical forces within this structure, or the generation of a high-frequency mechanical wave, may appear.

[0032] - a system for detecting a defect in this structure, this system comprising

[0033] - the above-mentioned device for measuring high-frequency mechanical waves propagating within the structure, this device comprises an optical fiber in which several measurement Bragg gratings are implemented, these measurement Bragg gratings being fixed without any degree of freedom to the structure, and

[0034] - a unit for monitoring the appearance of a defect in the structure from the high frequency mechanical waves measured by this device.

[0035] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the drawings in which:

[0036] - Fig. 1 is a schematic illustration of the architecture of an insulating structure equipped with a high-frequency acoustic wave measuring device,

[0037] - [Fig.2] is a schematic illustration of a measurement peak and an emission peak used in the measuring device of the [Fig.1],

[0038] - Figures 3 and 4 are schematic illustrations of a reader of the device measurement of [Fig.1],

[0039] - [Fig. 5] is a flowchart of a process for operating the structure trumented of the [Fig.l].

[0040] In this description, the terminology, conventions, and definitions of the terms used in this text are introduced in Chapter I. Detailed examples of embodiments are then described in Chapter II with reference to the figures. Variants of these embodiments are presented in Chapter III. Finally, the advantages of the different embodiments are specified in Chapter IV.

[0041] Chapter I: Definitions, terminology and conventions:

[0042] In the figures, the same references are used to designate the same elements.

[0043] In the remainder of this description, the well-known characteristics and functions of a person skilled in the art are not described in detail.

[0044] In this text, the symbol “*” denotes the scalar multiplication operation.

[0045] The term "high-frequency mechanical wave" or simply "mechanical wave" refers to a mechanical wave whose fundamental frequency is greater than 1 kHz and, preferably, greater than 10 kHz. Typically, it is an acoustic wave.

[0046] An ultrasonic wave is an acoustic wave whose frequency is greater than 20 kHz.

[0047] A “reflection power spectrum” or simply “reflection spectrum” is the power spectrum of the optical signal reflected by a Bragg grating.

[0048] An "emission spectrum" is the power spectrum of an optical source that emits an optical signal.

[0049] A peak in the reflection spectrum corresponds to an absorption line in the transmission power spectrum of the same Bragg grating.

[0050] The "width" of a peak in a power spectrum refers to its full width at half maximum (FWHM).

[0051] The fundamental wavelength XB of a Bragg grating is defined by the following relation: XB = 2*ne*A, where:

[0052] - ne is the effective index of the optical fiber in which the network is implemented Bragg, and

[0053] - A is the step size of the Bragg grating.

[0054] The effective propagation index ne is also known as the "mode phase constant". It is defined by the following relation: ng = neff - Xdneff / dk, where ng is the group index and X is the wavelength of the optical signal guided by the optical fiber. The effective propagation index of an optical fiber depends on the dimensions of the core of that optical fiber and the materials forming this core and the optical cladding. This optical fiber. It can be determined experimentally or by numerical simulation.

[0055] Chapter II: Examples of embodiments

[0056] Figure 1 represents an instrumented structure 2 comprising:

[0057] - a structure 6, and

[0058] - a system 8 for detecting a defect in the structure 6.

[0059] Structure 6 is a structure in which a defect, capable of altering the propagation of ultrasonic waves in that structure, may appear. The defect detectable using the system described here is, for example, a crack or a microcrack. It may also be a defect such as a trace of corrosion or a local modification of the structure's porosity.

[0060] Structure 6 is a mechanical part. By way of illustration, structure 6 is a thin structure. For example, here, the thin structure 6 has an outer face and an inner face separated from each other by the thickness e6 of the thin structure 6. The thickness e6 is small enough that the outer and inner faces guide the propagation of an elastic wave or a Lamb wave in the thin structure in directions parallel to these outer and inner faces. For this purpose, typically, the thickness e6 is ten or one hundred times smaller than a length and / or a width of the thin structure 6. Here, the thin structure 6 is a composite panel constituting the fuselage of an aircraft.

[0061] For example, the thin structure 6 is made of laminated composite materials, that is, by stacking, in a direction perpendicular to the outer face, a succession of layers, each made of a respective material. As explained in Chapter III on variants, the lessons learned in this particular case can be easily applied to many other possible structures.

[0062] For the sake of simplicity in [Fig. 1], the thin structure 6 is represented as a simple rectangle. However, in reality, the shape of the thin structure 6 is more complex. In particular, in the specific case of a composite panel of an aircraft fuselage, the thin structure 6 typically has rounded curves.

[0063] System 8 makes it possible, in particular, to detect the appearance of a defect by measuring the ultrasonic waves propagating in the structure 6. To this end, system 8 comprises:

[0064] - a device 10 for measuring ultrasonic waves propagating in structure 6,

[0065] - a transmitter 12 capable of emitting a predefined ultrasonic wave which propagates in structure 6, and

[0066] - a monitoring unit 14 which monitors the occurrence of a defect in the structure 6 from the measurements of device 10.

[0067] Device 10 comprises:

[0068] - an optical fiber 20,

[0069] - Bragg gratings measuring Bmj to BmN made one after the other along the axis of optical fiber 20, in its core, and

[0070] - a reader 22 capable of measuring the fundamental wavelengths of Bm gratings, at BmN.

[0071] The fiber 20 includes a sensitive portion 24 in which the Bmj to BmN gratings are formed. This sensitive portion 24 is fixed, without any degrees of freedom, to the structure 6 so that the ultrasonic waves propagating in the structure 6 are transmitted to the fiber 20 and modify the fundamental wavelength of the Bm, to BmN gratings when these ultrasonic waves encounter these Bmj to BmN gratings. For example, the portion 24 is bonded to one face of the structure 6. The wavy lines that separate the sensitive portion 24 into two segments indicate that only a part of this sensitive portion is shown in [Fig. 1].

[0072] A proximal end of the fiber 20 is optically connected to the reader 22.

[0073] Hereafter, the symbol “Bm;” denotes the i-th Bragg grating realized in the Optical fiber 20, where the index i equals one, corresponds to the Bragg grating closest to the reader 22. Here, N is equal to the total number of gratings Bm; implemented in fiber 20. Each grating Bm; has a fundamental wavelength λH at which a reflection peak appears in the reflection spectrum of that grating Bm;. This reflection peak of the grating Bm; is subsequently called the "measurement peak" and denoted "Pm;" to distinguish it from other reflection peaks of other Bragg gratings described later. The portion of the reflection spectrum of the grating Bm; that contains its peak Pm; is shown in [Fig. 2]. In [Fig. 2], the x-axis is graduated in nanometers and the y-axis is graduated in arbitrary units (aU). The peak Pm; of the grating Bm; is represented by a Gaussian curve. The peak of the Pm; curve is centered on the wavelength XBi.On either side of this peak, the curve Pm; has, from left to right, a rising edge Fm; then a falling edge Fd;. The edges Fm; and Fd; are approximately linear at mid-height of the peak Pm;. The peak Pm; is higher than the background noise within a range [XBi- ΔBi ; XBi+ ΔBi] of wavelengths. This range [XBi- ΔBi ; XBi+ ΔBi] is subsequently called the "reflection range" because an optical signal incident on the grating Bm; whose wavelength falls within the range [XBi- ΔBi ; XBi+ ΔBi] is, at least partially, reflected by the grating Bm;. Conversely, if the incident optical signal is completely outside this range [XBi- ΔBi ; XBi+ ΔBi], this optical signal is not reflected by the grating Bm;. This range [XBi- ΔBi; XBi+ ôBi] moves at the same time as the wavelength XBi changes.

[0074] The wavelengths / .H are all different from each other and spaced from each other so that the reflection ranges of each of the gratings Bm; do not They don't overlap.

[0075] The reader 22 interrogates, for example simultaneously, each grating Bm; to measure the optical signal reflected S;(t) by each of the gratings Bm;. A detailed embodiment of this reader 22 is described with reference to Figures 3 and 4.

[0076] The emitter 12 is controlled by the unit 14 to emit, typically at regular intervals, a predefined ultrasonic wave that propagates through the structure 6 until it reaches each of the gratings Bm. The ultrasonic wave generated by the emitter 12 is typically an elastic wave. In the case of a thin structure, this elastic wave is, for example, a Lamb wave. For example, the emitter 12 is a piezoelectric actuator fixed, without any degrees of freedom, to the face of the structure 6.

[0077] The monitoring unit 14 acquires measurements from the device 10 and, based on these measurements, detects the occurrence of a fault if such a fault appears in the structure 6. Thus, the unit 14 makes it possible to monitor the health status of the structure 6 and to inform a maintenance operator. To this end, the unit 14 comprises an electronic computer 30 and a human-machine interface 32 connected to the computer 30.

[0078] The computer 30 includes a programmable microprocessor 34 and a memory 36. The memory 36 contains the instructions and data necessary for the execution of the process in [Fig. 5], when these instructions are executed by the microprocessor 34. For example, here, for each network Bm;, the memory 36 contains a pre-recorded reference signal Sref;(t). This signal Sref;(t) is ultrasonic ground emitted by the transmitter 12 and measured using the network Bm; in the absence of a defect in the structure 6.

[0079] The human-machine interface 32 is capable of communicating, in a way directly intelligible to a human being, the results of the implementation of the detection process [Fig.5]. For example, the interface 32 includes a screen.

[0080] To simplify the description of reader 22, in [Fig. 3], only the optical components of reader 22 associated with a single grating Bm; are shown. The optical components associated with this single grating Bm; are those used to measure the ultrasonic wave at the location of this grating Bm;. In [Fig. 4], the optical components associated with several gratings Bm; of reader 22 are shown simultaneously. More precisely, [Fig. 4] represents the gratings Bm; with indices one to three and only the optical components associated with these first three gratings Bm;. In [Fig. 4], the optical components already described with reference to [Fig. 3] in the case of grating Bm; bear the same numerical references except that the index i is replaced by the indices one to three. Thus, in [Fig. 4], the optical components bearing the indices "1", "2", and "3" are those associated with the gratings, respectively Bmb, Bm2, and Bm3.Similarly, in the rest of this description, all components or peaks that bear the same index "i" are specifically associated with the Bm network; and therefore used for . acquire the ultrasonic wave that deforms this Bm network;.

[0081] Reader 22 comprises:

[0082] - a generator 50 of an excitation signal for the Bmi5 networks

[0083] - a demultiplexer 52 which extracts the optical signals reflected by each of the Bm networks;,

[0084] - a set of 54 photodiodes that each measure the optical signals extracted by the 52-bit multiplexer, and

[0085] - a device 56 for acquiring each of the signals measured by the set 54 of pho todiodes.

[0086] The generator 50 emits, on an output 60, an optical excitation signal whose emission spectrum comprises as many emission peaks Pg; as there are gratings Bm;. A peak Pgi is represented in [Fig. 2]. Each peak Pg; lies, at least partially, within the reflection range of the peak Pm;. This peak Pg; is centered on a wavelength XGi. The width of the peak Pg; is such that when the wavelength XH varies, the power of the optical signal reflected by the grating Bm; also varies. Preferably, the width of this peak Pg; is equal to or less than the width of the peak Pm;. For example, here, the width of each peak Pg; is half the width of the peak Pm;. The width of the peak Pgi is, however, preferably greater than 0.1 nm to obtain an incoherent light emission peak.

[0087] The wavelength XGi is located in the middle of one of the rising edges Fm and Fd. For example, in [Fig. 2], the wavelength XGi is located in the middle of the rising edge Fm, that is, at a location where the slope of the rising edge is maximum and substantially linear. Under these conditions, a slight displacement of the wavelength XBi of the grating Bm, caused by an ultrasonic wave, results in a variation of the optical signal reflected by the grating Bm, large enough to be detected by the photodiodes Pd (Figures 3 and 4). This is illustrated in [Fig. 2], where the symbol Si denotes a slight waviness of the wavelength λH and the symbol Sp denotes the corresponding waviness of the optical signal reflected by the grating Bm.

[0088] To produce this excitation signal, the generator 50 comprises:

[0089] - an optical source 62 which emits incoherent light onto an output 64, and

[0090] - a tunable filtering device 66 comprising an optically accessible input 68 connected to output 64 and an output that is confused with output 60 of the generator.

[0091] The source 62 emits incoherent light whose power spectrum includes a continuous emission band that extends over all the reflection ranges of the peaks Pm;. Thus, for all wavelengths within this emission band, the power of the incoherent light is greater than the background noise and therefore capable of producing a reflected optical signal when it encounters one of the gratings Bm;.

[0092] The filtering device 66 filters the incoherent light emitted by the source 62 to form each of the peaks Pg;. To this end, for each of the peaks Pg, the filtering device 66 has a -3 dB bandwidth centered on the wavelength XGi and whose width is equal to the width of this peak Pg;.

[0093] For this purpose, the filtering device 66 includes an optical circulator 72 and, for each network Bm;, a source Bragg network Bs;.

[0094] The circulator 72 comprises a first input optically connected to the input 68, a second input 74 optically connected to one end of an optical fiber 80 in which each of the networks Bs are carried out, and an output optically connected to the output 60. This circulator 72:

[0095] - directs the incoherent light received at input 68 only towards its second input 74, and

[0096] - directs the optical signal received on its second input 74 only to the output 60.

[0097] Thus, the optical excitation signal emitted on the output 60 is formed only by the optical signals reflected by the Bs networks; made in the fiber 80.

[0098] The Bs; gratings are arranged one after the other along the axis of the fiber 80. Each Bs; grating is configured so that its reflected spectrum includes the Pg; peak, that is, the peak centered on the XGi wavelength. Here, the Pg; peak of the Bs; grating is centered on the fundamental wavelength of the Bs; grating. To achieve this, typically, the pitch and number of motifs of the Bs; grating are adjusted to obtain this Pg; peak in its reflected spectrum.

[0099] The output 60 of the generator 50 is optically connected to a first input 90 of an optical circulator 92. The circulator 92 has a second input 94 optically connected to the end of the fiber 20 and an output 96 optically connected to an input 98 of the demultiplexer 52. The circulator 92:

[0100] - directs the excitation signal received at its input 90 only to its input 94, and

[0101] - directs the optical signal received on its input 94 only to the output 96.

[0102] Thus, only the optical signals reflected by the Bm networks are received by the demultiplexer 52.

[0103] The demultiplexer 52 extracts the optical signals reflected by each of the gratings Bm; and transmits each of the extracted optical signals to a respective photodiode Pd; of the array 54. The demultiplexer 52 includes for this purpose a series of tunable bandpass filters Fp;. Each filter Fp; has a -3 dB bandwidth [Xfi- ôfi ; Xfi+ ôfi] centered on a central wavelength Xfi and whose width 2*ôfi is greater than the width of the reflection range [XBi- ôBi ; XBi+ ôBi] of the peak Pm;. The wavelength and the range [Xfi- ôfi ; Xfi+ ôfi] are shown in [Fig. 2]. The width 2*ôfi is also sufficiently narrow so as not to overlap any other measurement peak. For example, here, the width 2*ôfi is greater than or equal to 4*ôBi or 6*ôBi and in- less than 10*ôBi or 20*ôBi.

[0104] Here, each filter Fp; comprises an optical circulator Co; and a filtering Bragg network Bf;.

[0105] The Bf gratings are fabricated in an optical fiber 110. More precisely, these Bf gratings are fabricated, for example, in ascending order of index i, one after the other along the axis of this fiber 110. Each Bf grating is shaped so that its reflected spectrum has a filtering peak Pf centered on the central wavelength Xfl. The width of this peak Pf is equal to 2πfl. Here, the peak Pf of the Bf grating is centered on the fundamental wavelength of the Bf grating. To achieve this, typically, the pitch and number of patterns of the Bf grating are adjusted to obtain this peak Pf in its reflected spectrum.

[0106] The circulator Co; of each filter Fp; is located just upstream of the network Bf;, that is to say on the side of the inlet 98 with respect to the network Bf;. Thus, with the exception of the circulator Coi which is interposed between the inlet 98 and the network Bfb each circulator Co; is interposed between the networks Bf; i and Bf;.

[0107] The circulator Co; comprises a first input Eh directly optically connected to input 98 for circulator Coi and directly optically connected to network Bf i for the other circulators Co;, a second input E2; optically connected to network Bf and an output Si optically connected to the photodiode Pd;. This circulator Co;:

[0108] - directs the optical signal received at its input E1; only to its input E2;, and

[0109] - directs the optical signal received on its input E2; only to its output Si.

[0110] When the input El; is connected to the previous network Bf H, this input receives the The optical signal that has passed through the Bf to Bf gratings receives the portion of the optical signal reflected by the Bf gratings that lies outside the reflection bands [Xf- Δf; Xf+ Δf] of the previous Bf to Bf gratings. The input E1 receives, in particular, the portion of the reflected optical signal that lies within the band [Xf- Δf; Xf+ Δf] of the Bf grating. This portion of the reflected optical signal is transmitted to the Bf grating via the E2 input. The Bf grating reflects only the portion of the reflected optical signal that lies within this band [Xf- Δf; Xf+ Δf]. The other portion of the reflected optical signal is transmitted to the E2 input of the Coi+i circulator. The portion of the optical signal that lies within the band [Xf- Δf; Xf+ Δf] is output from the Si output to the photodiode Pd.Thus, the photodiode Pd; only measures the reflected optical signal which is within the band [Xfi- ôfi ; Xfi+ ôfi] and therefore only the power of the peak Pm;. .

[0111] The acquisition device 56 acquires the electrical signals generated by each of the photodiodes Pd; and delivers to the computer 30 the digitized signals corresponding to each of these electrical signals. To this end, typically, for each photodiode Pd,, the acquisition device 56 includes an analog-to-digital converter CANi connected to the output of the photodiode Pd;.

[0112] To reduce the width of each band [Xfl- ôfi ; Xfl+ ôfi] while maintaining the ability to use the device 10 under potentially varying environmental conditions, the reader 22 further includes a servo device 130. This device 130 modifies the wavelength Xfl of each grating Bf; to cancel variations in the DC component of the optical signal reflected by the grating Bm; associated with this grating Bf;. The DC component is generated by slow variations in the wavelength XBi, that is, variations much slower than those caused by an ultrasonic wave. These slow variations in the wavelength XBi are typically those caused by variations in environmental conditions. Indeed, environmental conditions vary at a frequency below 500 Hz and, typically, below 100 Hz or 10 Hz or 1 Hz.

[0113] For this purpose, the device 130 includes a microcontroller 132 and a mechanical transducer Tm; for each network Bf;.

[0114] The transducer Tm; is capable of mechanically deforming the grating Bf; without deforming the other filtering Bragg gratings. The transducer Tm; directly deforms the grating Bf; without requiring local heating. For this purpose, the transducer Tm; is a piezoelectric material to which the grating Bf; is fixed, without any degrees of freedom. For example, the grating Bf; is bonded to one face of this piezoelectric material.

[0115] The microcontroller 132 receives the electrical signals generated by each of the photodiodes Pdi and estimates, from each received electrical signal, the value of the DC component of the optical signal reflected by each array Bm;. For example, here, the cumulative power of the components of the electrical signal located, in the power spectrum, at frequencies below 100 Hz is used as the representative value of the DC component. Then, based on the estimated value of the DC component, the microcontroller 132 generates a command for the transducer Tm; which aims to compensate for variations in this DC component and thus maintain the wavelength Xfl centered on the wavelength XBi. In other words, the command generated by the microcontroller 132 cancels out variations in the measured DC component.To achieve this, the microcontroller 132 executes a control algorithm that controls the estimated DC component value against a predetermined setpoint. Typically, the predetermined setpoint is the estimated DC component value in the absence of an ultrasonic wave and when the wavelength is correctly centered on the XBi wavelength. For example, this setpoint is measured during a preliminary calibration phase. The control algorithm is, for example, a PID (Proportional, Integral, Derivative) controller. Thus, the central wavelength Xfl is automatically shifted to compensate for variations in the DC component of the optical signal reflected by the grating Bm. Under these conditions, the wavelength Xf shifts like the wavelength XBi when this shift in wavelength is caused by a change in environmental conditions.

[0116] Furthermore, here, the same control device 130 is also used to control the wavelength XGi of each grating Bs; to the DC component of the optical signal reflected by the grating Bm; associated with this grating Bs;. This makes it possible to maintain the relative position of the wavelength XGi with respect to the wavelength XBi substantially constant when the wavelength XBi varies in response to a change in environmental conditions. For this purpose, here, the grating Bs; is also fixed without any degrees of freedom to one face of the piezoelectric material of the transducer Tm;.

[0117] In [Fig.4], to simplify the representation of reader 22, the device Acquisition unit 56 and microcontroller 132 were not shown. Furthermore, in [Fig. 4] only a portion of the input / output digital references of the optical components were included.

[0118] The operation of system 8 will now be described with reference to the process in [Fig.5].

[0119] During an instrumentation step 200, the Bm network; are each fixed to a respective location on the structure 6.

[0120] Then, in a step 202, the reference signal Sref;(t) is recorded for each of the networks Bm;. For example, for this purpose, in the absence of a fault in the structure 6, an ultrasonic wave is measured, using the device 10, by each of the networks Bm;. It is this ultrasonic wave measured in the absence of a fault, using the network Bm;, that is then recorded in the memory 36 as the reference signal Sref;(t) associated with this network Bm;. In this step 202, each signal Sref;(t) is measured in the same way as during the operating phase of the fault detection system 8.

[0121] Next, a phase 210 of the operation of the fault detection system 8 is executed. During this phase 210, the following steps are repeated at regular intervals.

[0122] During a step 212, the computer 30 commands the transmitter 12 to emit a predefined ultrasonic wave into the structure 6.

[0123] In parallel, during a step 214, the generator 50 continuously emits the excitation signal into the fiber 20. Each network Bm; then reflects the part of the excitation signal that corresponds to the peak Pg;.

[0124] Also in parallel with step 214, during step 216, the demultiplexer 52 extracts each optical signal reflected by a particular grating Bm; and directs it to the corresponding photodiode Pd;. Thus, during step 216, each photodiode Pd; transforms the extracted optical signal into a corresponding electrical signal.

[0125] As the photodiodes Pd; generate the electrical signals, in a step 218, the acquisition device 56 transforms them into digital signals Si (t) which are transmitted to the computer 30.

[0126] Here, the electrical signals S;(t) are also transmitted to the servo device 130 which, in a step 220, uses the DC component of each of these electrical signals to control a shift in the wavelengths XGi and Xfide so that these wavelengths XGi and Xfl follow the shift in wavelength XB; which is caused by a variation in environmental conditions. This shift in wavelengths XGi and / .(. thus compensates for the variation in environmental conditions.

[0127] Once the complete S;(t) signals have been acquired by the computer 30, in a step 230, the computer 30 compares each S;(t) signal to the Sref;(t) signal to deduce the existence of a fault in the event of a significant difference between these two signals. If a fault is detected, the computer 30 commands the interface 32 to transmit this information to an operator.

[0128] Chapter III: Variants:

[0129] Variants of the structure:

[0130] The measuring device described herein applies to thin structures other than an aircraft fuselage panel. For example, the thin structure may also be a plate, a rail, a tube, a bar, or any other part whose thickness is small compared to its length or width. In particular, for example in the case of a bar, the thin structure does not necessarily have both an external and an internal face.

[0131] The structure is not necessarily a thin structure. For example, the structure may be a civil engineering work such as a bridge or a roadway on which a vehicle travels. In this case, the emitted signal is adapted to propagate, without being attenuated too much, within the structure and, preferably, parallel to a face of that structure. For example, for this purpose, a Lambda wave is replaced by a Rayleigh wave that propagates parallel to a face of the structure.

[0132] The structure can be made of materials other than a laminated composite material. For example, the structure can be made of a non-laminated or non-composite material. In this case, for example, the structure is a turbine or propeller blade. Thus, the measuring device described here can also be used with structures made of metal or concrete.

[0133] Generator variants:

[0134] The emission peak Pg; may be wider. For example, the width of peak Pg; is equal to the width of peak Pm;. In another embodiment, the width of peak Pg; is su greater than the width of the peak Pm. In all cases, the width of the peak Pg is small enough that a variation in the wavelength XBi results in a variation in the power of the optical signal reflected by the grating Bm.

[0135] Alternatively, the generator 50 is further equipped with a sensor that measures the source signal reflected by each of the arrays Bs;. This source signal measured for each of the arrays Bs; is acquired by the computer 30. During the processing of the optical signal reflected by one of the arrays Bm;, the computer normalizes the power of the reflected optical signal. For this purpose, for example, the power of the reflected optical signal is normalized by dividing it by the power of the source signal measured for the array Bs;. This makes it possible to compensate for some of the noise in the excitation signal.

[0136] The generator 50 is not necessarily an incoherent light generator. For example, if the distance between each of the gratings Bm; and the generator 50 is short, the generator 50 can be replaced by a generator comprising as many laser sources as there are gratings Bm;. Each of these laser sources emits a coherent excitation beam at a respective wavelength λ.g. Each of these laser sources emits a monochromatic beam, that is, a beam corresponding to a peak Pg; whose width is less than 1 nm and, preferably, less than 0.5 nm. In this case, the noise of the excitation signal is lower than that obtained using an incoherent light source. The noise attenuation device for the generator can therefore be omitted.

[0137] In another embodiment, for each Bm grating, the generator comprises a source implemented using a phase-shifted Bragg grating and an amplifier that amplifies the signal reflected by the phase-shifted Bragg grating before reinjecting it. Such a source is described in detail, for example, in the following article: Laffont G. et al.: “Wavelength tunable fiber ring laser for high-speed interrogation of fiber Bragg grating sensors,” Proc. SPIE 5855, 17th International Conference on Optical Fibre Sensors, 23 May 2005.

[0138] In a simplified embodiment, the filtering device 66 is not tunable. In this case, in response to a change in environmental conditions, the power of the reflected optical signal varies. However, even in such a case, it remains possible to separate the high-frequency component, which corresponds to the measured ultrasonic signal, from the low-frequency component, which corresponds to the changes in environmental conditions. In such an embodiment, the width of the peak Pg; is chosen sufficiently large so that, in response to a change in environmental conditions, at least a part of this peak Pg; remains within the range [XBi- ô; ; W ÔJ.

[0139] Demultiplexer variants:

[0140] Other embodiments of the Fp filters are possible. In particular, in Alternatively, Fp filters are made without using Bragg gratings. For example, the Fpi filter can also be made from an absorbing or dichroic optical filter.

[0141] Alternatively, the Fp filters are made in several optical fibers connected to the same input 98 via an optical coupler.

[0142] Variants of the servo device:

[0143] Other embodiments of the transducers Tm; are possible. For example, the transducer Tm; comprises an electrical resistance that heats a material to which the networks Bs; and Bf; are fixed without any degrees of freedom. In this case, it is the thermal expansion of this material that mechanically deforms the networks Bs; and Bf;.

[0144] The servo device 130 can implement servo algorithms other than a PID controller. For example, alternatively, the microcontroller 132 implements PI (Proportional, Integral) controllers.

[0145] The preceding embodiments have been described in the case where the control device 130 is common to the networks Bf; and Bs;, that is, the same mechanical transducer and the same control algorithm are used to deform the networks Bf; and Bs; simultaneously and in the same way. However, alternatively, this common control device can be replaced by a first and a second control device. The first control device uses first transducers to mechanically deform each of the networks Bf; independently of each other. The second control device uses second transducers, different from the first transducers, to deform each of the networks Bs; independently of each other. Each first and each second transducer are typically piezoelectric materials that are mechanically distinct from each other.In this case, the control algorithm executed by the first control device may differ from the control algorithm executed by the second control device. To achieve this, the first and second control devices each receive, as input, the electrical signal delivered by each photodiode Pd. In this case, where there are two independent control devices, one of them may be a master control device while the other is a slave control device. Thus, in this latter case, the first transducers are mechanically independent of the second transducers, but the control algorithms of the first and second control devices are not necessarily so.

[0146] Other variants of the measuring device:

[0147] Other methods of fixing the sensitive portion 24 of the fiber 20 to the structure 6 are possible. For example, if the thickness of the structure 6 is sufficient, the sensitive portion 24 is integrated, without any degree of freedom, into the thickness of this structure during its manufacture.

[0148] Alternatively, the device 10 does not comprise a single optical fiber 20 but several optical fibers 20, each of these optical fibers 20 comprising several Bragg gratings Bm; . In this case, what has been described here for the optical fiber 20 applies to each of these optical fibers. This alternative can be transposed to the case of a multi-core fiber. In the latter case, several measurement Bragg gratings are implemented in each of the cores of this multi-core fiber.

[0149] Each optical circulator can be replaced by a set of optical couplers and optical isolators which performs the same function.

[0150] Variants of the method for detecting a defect:

[0151] Processing methods other than comparing the ultrasonic ground measured by a Bm grating to the reference signal Sref(t) are possible. For example, alternatively, the processing performed to detect a fault from measurements of a Bm grating uses vibro-acoustic modulation. Such processing is described in detail in EP4155724. In this case, it is not necessary to use a pre-recorded reference signal.

[0152] In other variants, emitter 12 is omitted. In this case, the Bm networks are used to measure the acoustic background generated by the defect itself when it appears in the structure. In this case, emitter 12 can be omitted. Such a process for identifying a defect from the acoustic waves generated by the defect itself is, for example, described in application RU2737235C1.

[0153] Alternatively, the measurements of the device 10 can also be used to determine the time intervals that have elapsed for an acoustic wave to propagate from the location of the defect to the locations of the Bm networks. Then, knowing the positions of each of the Bm networks, these determined time intervals are used, for example, to estimate the position of the defect in the structure.

[0154] Other variants:

[0155] The apparatus 10 can also be adapted to measure mechanical waves with frequencies between 1 kHz and 20 kHz. In particular, these acoustic waves with frequencies between 1 kHz and 20 kHz can also be used to detect defects in structures.

[0156] The microprocessor 34 can be a generic processor, a specific processor, an application-specific integrated circuit (also known as an ASIC for "Application-Specific Integrated Circuit") or an in situ programmable gate array (also known as an FPGA for "Field-Programmable Gate Array").

[0157] Several of the variants described above can be combined in the same embodiment.

[0158] Chapter IV: Advantages of the embodiments described:

[0159] Varying the central wavelength Xfl of each bandpass filter Fp, to cancel the DC component of the optical signal reflected by each grating Bm, reduces the width of each passband [Xfl- θfi ; Xfl+ θfl], compared to the case where the wavelengths Xfl are constant. Indeed, when the wavelengths Xfl are constant, the width of each passband [Xfl- θfi ; Xfl+ θfl] must be sufficiently large to always include the peak Pm, even if the wavelength θH varies depending on environmental conditions. Therefore, the passband [Xfl- θfi ; Xfl+ θfl] must be widened so that even when extreme environmental conditions are encountered, the peak Pm remains within this passband.Conversely, when the wavelength Xfl is modified according to the DC component of the optical signal reflected by the grating Bm;, the wavelength Xfi shifts like the wavelength XBi in response to variations in environmental conditions. Thus, the bandwidth [Xfi- ôfl ; Xfi+ ôfi] shifts according to the environmental conditions encountered, always keeping the peak Pm; within this bandwidth. Therefore, the bandwidth width [Xfi- ôfi ; Xfi + ôfi] can be much smaller since it does not depend on the extreme environmental conditions that may be encountered. Furthermore, being able to reduce the bandwidth width [Xfi- ôfi ; Xfi+ ôfi] of each filter Fp; makes it possible to reduce the gap between successive peaks Pm;. Therefore, this makes it possible to increase the density of Bm networks; that is to say, within the same spectral band, it is possible to use a larger number of them.

[0160] The use of a Bragg filtering network Bf; to realize the filter Fp; simplifies the realization of this bandpass filter.

[0161] The use of a transducer Tm, which directly mechanically deforms the grating Bf, to vary the wavelength Xfl allows this wavelength Xfl to be varied very rapidly. This improves the accuracy of the measurement.

[0162] Using incoherent light and the filtering device 66 to form the excitation signal, instead of using laser sources, makes it possible to obtain an excitation signal that generates less noise when propagating in the fiber 20. Thus, this optical excitation signal makes it possible to interrogate Bm networks; located more than one kilometer from the generator 50. The generator therefore makes it possible to interrogate Bm networks; over greater distances than when a coherent laser beam is used to do this.

[0163] The use of the Bs network; to generate the source peak Pg; allows this source peak to be generated simply.

[0164] The fact that the width of the source peak Pg; is equal to or less than the width of the measurement peak Pm; increases the sensitivity of the device and avoids areas of loss of sensitivity.

[0165] Varying the center wavelength XGi of each grating Bs; to cancel variations in the DC component of the optical signal reflected by the grating Bm; allows the device 10 to function correctly even if the grating Bm; is subjected to environmental conditions which slowly vary its wavelength XBi.

[0166] The use of a mechanical transducer Tm; to vary the wavelength / . Gi allows this central wavelength to be varied very rapidly. This improves the accuracy of the measurement.

[0167] Using the same transducer Tm; to mechanically deform both the Bs; and Bf; network simplifies the realization of the device 10.

[0168] Using a piezoelectric material as a transducer simplifies the implementation of the control device.

Claims

Demands

1. Apparatus for measuring high-frequency mechanical waves, i.e., mechanical waves with frequencies above 1 kHz, this apparatus comprising: - an optical fiber (20) in which several measurement Bragg gratings (Bm;) are made, intended to be exposed to high-frequency mechanical waves, each of these measurement Bragg gratings having a reflection spectrum which includes a measurement peak (Pm;) centered on a measurement wavelength (XBi) and whose width defines a reflection range which contains all the wavelengths reflected by this measurement peak, the measurement wavelengths (XBi) of each of these Bragg gratings being different from each other and spaced from each other so that the different reflection ranges of these measurement Bragg gratings are distinct and do not overlap, - a generator (50) capable of emitting an optical excitation signal in the optical fiber, the emission spectrum of this optical excitation signal comprising as many emission peaks (PgJ) as there are measurement Bragg gratings, each of these emission peaks being located, at least in part, within the reflection range of a respective measurement peak, - a demultiplexer (52) capable of extracting the optical signals reflected by each of the measurement Bragg gratings, this demultiplexer comprising for this purpose a series of bandpass filters (Fp;) in which each bandpass filter (FpO) is associated with a respective measurement Bragg grating (Bm;), each of these bandpass filters having a -3 dB bandwidth centered on a central wavelength (X fi) and the width of this bandwidth being greater than the reflection range of the measurement peak of the measurement Bragg grating to which it is associated and sufficiently narrow so as not to encroach on a measurement peak of another measurement Bragg grating, and - a set (54) of photodiodes (Pd;) capable of measuring each of the optical signals extracted by the demultiplexer, characterized in that the device comprises a first servo device (130) capable of varying the central wavelength (Xfi) of each bandpass filter (Fp;) to cancel the variations in the DC component of the optical signal reflected by the Bragg grating of measurement to which it is associated.

2. Apparatus according to claim 1, wherein each bandpass filter (Fpi) comprises a filtering Bragg grating (Bf;) optically connected to the optical fiber (20) to receive the optical signal to be demultiplexed and optically connected to one of the photodiodes to deliver to that photodiode the optical signal extracted by that bandpass filter, the reflection spectrum of that filtering Bragg grating comprising a filtering peak (Pf;) centered on the central wavelength (Xfl) of that bandpass filter and the width of that filtering peak being greater than the width of the reflection range of the measurement peak (Pm;) of the measurement Bragg grating associated with that bandpass filter.

3. Apparatus according to claim 2, wherein the first servo device (130) comprises, for each filtering Bragg grating (Bf; ), a transducer (¾) capable of mechanically deforming this filtering Bragg grating to modify the central wavelength (Xfi) of this bandpass filter.

4. Apparatus according to any one of the preceding claims, wherein the generator comprises: - an optical source (62) capable of emitting on an output (64) incoherent light whose power spectrum includes an emission band that extends over all the reflection ranges of the measurement peaks (Pm;), and - a filtering device (66) comprising an input (68) optically connected to the output of the optical source and an output optically connected to the optical fiber (20), the power spectrum of this filtering device comprising as many -3 dB bandwidths as there are measurement Bragg gratings, each of these bandwidths being centered on a respective emission peak (Pg;) to form this emission peak in the optical excitation signal.

5. Apparatus according to claim 4, wherein the filtering device (66) comprises as many source Bragg gratings (Bs;) as measurement Bragg gratings (Bm;), each source Bragg grating being associated with a respective measurement Bragg grating, the reflection spectrum of each source Bragg grating comprising the emission peak (Pg;) which lies, at least in part, within the reflection range of the measurement peak of the measurement Bragg grating associated with it.

6. Apparatus according to claim 5, wherein the emission peak width (Pg;) of each source Bragg grating (Bs;) is equal to or smaller than the width of the measurement peak of the measurement Bragg grating associated with it.

7. Apparatus according to claim 5 or 6, wherein each emission peak (Pg;) is centered on a central wavelength (XGi) and the apparatus includes a second servo device capable of varying the central wavelength (XGi) of each source Bragg grating to cancel variations in the DC component of the optical signal reflected by the measurement Bragg grating to which it is associated.

8. Apparatus according to claim 7, wherein the second control device comprises, for each source Bragg grating, a mechanical transducer capable of mechanically deforming this source Bragg grating to modify the central wavelength (XGi) of this source Bragg grating (Bs;).

9. Apparatus according to claims 3 and 8 taken together, wherein the first and second servo devices (130) are combined and each transducer (Tm;) is capable of mechanically deforming both the filtering Bragg grating (Bf) of the bandpass filter (Fp;) and the source Bragg grating (Bs;) which are associated with the same measuring Bragg grating (Bm;).

10. Apparatus according to claim 9, wherein each transducer (TmO) is a piezoelectric material on which the filtering Bragg grating (Bf) and the source Bragg grating (Bs;) are fixed without any degree of freedom.

11. An instrumented structure comprising: - a structure (6) in which a defect, capable of modifying the propagation of high-frequency mechanical waves in this structure or of generating a high-frequency mechanical wave, may appear, - a system (8) for detecting a defect in this structure, this system comprising - an apparatus (10) for measuring the high-frequency mechanical waves propagating in the structure, this apparatus comprising an optical fiber (20) in which several measurement Bragg gratings (Bm;) are formed, these measurement Bragg gratings being fixed without any degrees of freedom to the structure, and - a unit (14) for monitoring the appearance of a defect in the structure based on the high-frequency mechanical waves measured by this apparatus, characterized in that the apparatus (10) for measuring the mechanical waves high frequencies complies with any one of the preceding claims.

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